This invention relates generally to obstacle detection and ranging systems and more specifically to near range obstacle detection systems. The following are a few of the applications in which such a near range obstacle detection system may be used:
Vehicular obstacle detection and headway control PA1 Autonomous tele-operated vehicle obstacle detection PA1 Space robotics PA1 Control of work platforms and forklifts PA1 Terrain mapping through vegetation PA1 Weapons fusing PA1 Battlefield surveillance PA1 Tank gauging (determining the amount of a substance stored in a container) PA1 Marine vessel docking and guidance PA1 Airplane auto-docking PA1 Personnel bridge docking PA1 Airport runway incursion PA1 Altimeter PA1 Presence sensor four traffic light control PA1 Ice thickness measurement PA1 Pavement thickness measurement PA1 Buried object detection PA1 Underground tunnel or void detection PA1 Perimeter security surveillance PA1 Aid to the handicapped
Some prior art near obstacle detection systems utilize infrared and ultrasonic radiation. These systems generally have disadvantages that discourage their use.
Microwave radiation on the other hand is commonly used in a variety of forms of radar systems, and the advantages of microwave radar technology make it attractive for near obstacle detection systems as well. See the article "Automotive Radar: A Brief Review" by D. M. Grimes and T. O. Jones in Proceedings of the IEEE, June, 1974, pp. 804-822 and the relevant prior art literature cited therein.
An important microwave operating band assigned for radar use, generally designated as X-band, covers the frequency range from 8.2 to 12.4 GHz. In this frequency range, microwave components are reasonable in both size and cost. For example, the dimensions of an X-band planar or patch antenna, suitable for near obstacle detection, are approximately 1 inch.times.2 inches. A portion of X-band set aside by the Federal Communications Commission for unlicensed use covers the frequency range from 10.50 to 10.55 GHz. However, this limited bandwidth makes it difficult to achieve adequate resolution for nearby targets. For example, with a conventional frequency modulated-continuous wave (FM-CM) radar system operating over a 50 MHz bandwidth, the minimum resolution is approximately 10 feet, whereas a resolution of the order of inches is considered necessary for near obstacle detection such as vehicular warning systems.
Where antenna mounting space is at a premium, it is possible to utilize one antenna for both transmitting and receiving. This one antenna system is called a monostatic radar system. A major drawback of monostatic systems is the unwanted presence of an internally reflected signal from the antenna. Since the internally reflected signal may be an order of magnitude larger than the reflected signal from the target, the accurate detection of a target may not be possible in a narrow band system since the receiver detects the composite signal consisting of the internally reflected signal from the antenna as well as the reflected signal from the target.
An analogous situation occurs in bi-static or two antenna systems. The unwanted signal is due to the leakage between the two antennas. However this leakage signal is usually much smaller than the reflected antenna signal in the monostatic system. In most situations, the leakage signal can be ignored. However, in some cases, an active two-port phase shifter/attenuator or I-Q modulator is employed. This two-port device generates a signal of equal magnitude and opposite sign to that of the leakage signal, cancelling the leakage signal.